The present invention relates generally to the fabrication of semiconductor lasers, and, more particularly, to both long and short wavelength vertical cavity surface emitting lasers (VCSEL) using Bragg reflectors buried in an indium phosphide (InP) material system or a gallium nitride (GaN) material system and a method for producing same.
Light emitting diodes (LED""s), lasers, and the like (collectively known as light emitting devices) are used widely in many applications today such as communications systems, medical systems, and display systems. These light emitting devices are commonly fabricated with epitaxial materials formed on a substrate, the epitaxial materials having a p-n junction, or active region, formed therein and typically include at least one Bragg reflector. A Bragg reflector is a fundamental building block of many light emitting devices. In LED""s, a Bragg reflector is fabricated between an active region and a substrate to reflect light out of the LED, and in lasers, a Bragg reflector is fabricated on either side of an active region to define an optical cavity. In the case of an LED, light emitted from an active region toward the substrate is reflected by a Bragg reflector back toward the surface where it combines with the light going toward the surface, thus increasing the light output of the LED. Bragg reflectors are typically composed of alternating layers of material having different refractive indices. In a semiconductor laser, one Bragg reflector should have a reflectivity approaching 99.9% and the opposing Bragg reflector (i.e., the one through which the laser light is emitted) should have a reflectivity of approximately 99.8%.
For lasers used in communication systems, and particularly for optical communications systems, it is desirable for the laser to emit a relatively long wavelength light on the order of approximately 1.3-1.55 micrometers (am) and emit that light in a single spatial mode and in a single longitudinal mode. Laser emission in a single spatial mode and in a single longitudinal mode results in laser emission at a single frequency. A long wavelength, single frequency output allows the laser emission to be focused into an optical fiber and to perform well in communications systems in which very high communication rates over long distances at high frequencies are required.
Laser emission at these preferred wavelengths requires that a laser be fabricated of a material having a small band gap, such as indium phosphide. Unfortunately, it is difficult to form Bragg reflectors using indium phosphide because it is difficult to find a compatible material having a refractive index different than that of indium phosphide, thus requiring an inordinately large number of reflector pairs in order to achieve the required reflectivity.
One type of laser device that is capable of emitting a single frequency output at the desired communication frequency is a distributed feedback laser (DFB). However, distributed feedback lasers tend to be high in cost.
Another way of achieving a long wavelength laser emission is to grow Bragg reflectors using a gallium arsenide (GaAs) material system and then join the GaAs Bragg reflectors, using a technique known as wafer bonding, to an indium phosphate (InP) substrate. A significant drawback to this method is that, typically, there is poor electrical conductivity across the bonded interface. Another drawback to this method is that the joining of the two different material systems using wafer bonding adds a costly manufacturing step, and requires the growth of two different material systems.
Yet another method for creating a long wavelength laser without the use of wafer bonding is to use a GaAs semiconductor laser with an active layer comprising, for example, gallium arsenide nitride (GaAsN) or gallium arsenide phosphide antimonide (GaAsPSb), however, the growth of these active layers is difficult, costly and time consuming.
Therefore, an unaddressed need exists in the industry for a long wavelength single frequency laser that may be fabricated simply from a single material system at a low cost.
The invention provides a semiconductor laser and method for producing same. Although not limited to these particular applications, the present invention is applicable to forming an indium phosphide (InP), long wavelength semiconductor laser and also, semiconductor lasers fabricated from the gallium nitride (GaN) material system.
In architecture, the present invention may be conceptualized as a semiconductor laser, comprising a substrate assembly containing a first reflector, a current confinement region located on a surface of the substrate assembly, an epitaxial lateral overgrowth layer over the current confinement region, and a second reflector deposited over the substrate assembly.
The present invention may also be conceptualized as providing a method for forming a semiconductor laser, comprising the following steps: forming a substrate assembly containing a first reflector, forming a current confinement region located on a surface of the substrate assembly, growing an epitaxial lateral overgrowth layer over the current confinement region, and forming a second reflector over the substrate assembly.
The invention has numerous advantages, a few of which are delineated, hereafter, as merely examples.
An advantage of the invention is that it allows a semiconductor laser to emit a light output at a single spatial mode and a single longitudinal mode, resulting in a light output at a single frequency.
Another advantage of the invention is that it improves the current confinement capability of a semiconductor laser.
Another advantage of the invention is that allows a long wavelength semiconductor laser to be fabricated from a single material system.
Another advantage of the invention is that it allows a long wavelength semiconductor laser to be fabricated using commonly available materials with which to create the active region.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.
Other features and advantages of the invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the present invention.